Chromium salts are known in which the oxidation state of chromium varies between one and six. Extensive investigation has shown, however, that chromium III is the most stable and important oxidation state of the element. An important characteristic of the chromium III ion is that it has six coordination sites arranged in an octahedral configuration about the central ion. The coordination sites of chromium III account for the existence of stable complex ions such as the hexaaquochromium ion Cr(H.sub.2 O).sub.6 .sup.+.sup.+.sup.+ and the hexaminochromium ion Cr(NH.sub.3).sub.6 .sup.+.sup.+.sup.+ . In both of the above examples the water and ammonia, commonly called ligands (L), occupy the six coordination sites of chromium III and are arranged in an octahedral configuration about the central chromium III ion. ##EQU1## Ligands may be electrically neutral, as in the cases of water and ammonia, or negatively charged as in the case of the cyanide ion which gives rise to the negatively charged hexacyanochromium ion Cr(CN).sub.6.sup..sup.-3.
Further, chelating agents, such as the acetylacetonate anion, form exceedingly stable chromium chelates in which all of the chromium III coordination sites are occupied. ##EQU2##
The removal of the above-mentioned ligands from the chromium III ion or the displacement of these ligands by other ligands is an extremely difficult and slow process. It is largely because of this kinetic inertness that so many complex chromium III species can be isolated and that they persist for relatively long periods of time in solution, even under conditions where they are thermodynamically quite unstable. Thus, the normally occurring form of chromium III compounds is the fully coordinated state. The kinetic stability of its widely found complex coordination compounds sets the chromium III ion apart from most other trivalent transition metal ions. We have found that the commonly occurring fully coordinated chromium III carboxylates are poor catalysts for carboxylic acid-oxirane reactions. Quite surprisingly, however, we have found that Chromium III compounds wherein coordination sites are readily available for coordination (complexing) by either charged or neutral ligands act as superior catalysts for such reactions and also for the acid-imide reaction.